Answer: Small cell carcinoma

Histology: This neoplasm has a nested growth pattern. The nests are associated with large areas of central necrosis. High power examination of tumor cells reveals coarse but evenly distributed chromatin, frequent mitoses, and a lack of prominent nucleoli. There are areas with nuclear molding, and the periphery does not show prominent nuclear palisading. These features are diagnostic of small cell (high grade neuroendocrine) carcinoma.

Discussion: Basaloid carcinoma is a key differential diagnosis. The current lesion shows nuclear molding, a lack of peripheral palisading, and a lack of reactivity for high molecular weight cytokeratin that would characterize a basaloid squamous carcinoma. A poorly differentiated adenocarcinoma should show some areas of gland formation. A carcinoid tumor would lack the extensive necrosis and high mitotic rate of the current lesion.

Resection specimens of small cell carcinoma, such as this one, often show a somewhat different morphology than the classic small cell carcinoma morphology seen on small transbronchial biopsy sections. In larger resection specimens, likely due to differences in specimen handling and fixation, the cells of small cell carcinoma appear larger and may show more cytoplasm. Nonetheless, the nuclear features of the neoplasm allow the diagnosis to be established.